The long-term goal of this research project is to understand the enzymatic and motor properties of myosin III that allow it to play a direct role in the phototransduction process in photoreceptors. Myosin III is an actinbased molecular motor that contains a conserved motor domain characteristic of the myosin superfamily, but also contains an N-terminal kinase domain as well as an additional actin-binding motif in its C-terminal tail. We propose that the ability of the kinase domain to autophosphorylate sites on the myosin domain reduces the enzymatic and motile properties of myosin III, but when the kinase domain is inactivated or bound to another substrate the myosin III motor is activated. We will examine the enzymatic and motile properties of myosin III constructs with the kinase domain deleted or inactivated, as well as in the presence of cellular substrates for the kinase domain. The kinase regulation of myosin III may provide a mechanism whereby myosin III can be activated/inhibited in response to specific cellular signals. We propose that the additional actin binding site in the tail allows myosin III to cross-link actin filaments, cooperatively activate the myosin III enzymatic cycle, and enhance the motile properties of myosin III by increasing the overall affinity of myosin for actin. We will directly examine the actin binding affinity of the tail actin-binding motif, and determine the role that it plays in mediating the enzymatic and motile properties of myosin III. Thus, the additional actin-binding site may allow myosin III localize to and help stabilize actin filament networks, which are necessary for assembling the proteins involved in the phototransduction signaling complex. Overall, studying the enzymatic, motor, and regulatory properties of myosin III may reveal potential mechanisms of how the formation and stabilization of the phototransduction signaling complex occurs in photoreceptors. [unreadable] [unreadable]